1. Field of the Invention
The present invention relates to a process for the production of homo- and co-polymers of .alpha.-olefins with the aid of catalyst systems containing microcrystalline titanium(III) compounds.
2. Discussion of Prior Art
The polymerization of .alpha.-olefins to the corresponding homo- and co-polymers by the use of Ziegler-Natta catalysts is employed on a large scale in the production of plastics having different desired technical properties. The Ziegler-Natta catalysts generally consist of a titanium(III) compound, which may be present either as a pure compound or as a crystalline solid solution containing also an aluminum compound, and an activator. Such catalyst systems usually are prepared by first reducing a titanium(IV) compound by means of organoaluminum or organomagnesium compounds to a titanium(III) compound. Suitable reducing agents are, for example, magnesium metal, Grignard compounds or magnesium dialkyls, or mixtures thereof, or aluminum metal, trialkylaluminum compounds or alkylaluminum chlorides, or mixtures thereof. After elimination of the dissolved reducing agents by washing, an organoaluminum compound of the type named above is added to the titanium(III) compound as activator.
When this general procedure is followed, the titanium(III) compound is always obtained in the form of relatively coarse particles having a diameter greater than 20.mu.. Now in some cases it is advisable to use the titanium component of the catalyst system in as finely divided form as possible since the smaller particle size will increase the activity of the catalyst and reduce the particle size of the polymers produced with it. Small particle size of the polymer is of importance in view of the further processing of the plastic. This is true especially of ultrahigh-molecular-weight polyolefins, in other words, polyolefins having a molecular weight, determined viscosimetrically, of over 500,000 and in particular of 1,000,000. Such polyolefins cannot be processed into shaped articles by means of the techniques usually employed with thermoplastics, such as extrusion or injection molding. They are therefore processed primarily by compression molding and sintering. In order to assure that such additives as colorants, antioxidants and UV stabilizers are uniformly distributed in the semifinished material and in the finished articles. The ultrahigh-molecular-weight polyolefins must be used in particularly finely divided form.
Many attempts have therefore been made to prepare catalysts of minute particle size. U.S. Pat. No. 2,968,652 for example, describes a method of comminuting titanium(III) compounds of a crystal size of about 25.mu. to a particle size ranging from about 0.1 to 5.mu. by ultrasonically treating the crystallized product in a neutral liquid medium. The minute particles so obtained are said to give higher yields of crystalline polymers in a shorter time than coarser catalysts do. However, the polymers produced with these catalysts are not of sufficiently small particle size, for which reason they must be subjected to a subsequent size reduction. Because of the equipment required, this method is not suited for use on an industrial scale.
Another process is described in German Pat. No. 15 95 661. There a TiCl.sub.3 is used as catalyst component in the production of particulate polymers which is prepared in the presence of a rubberlike hydrocarbon polymer incorporating active oxygen atoms which contains from 0.1 to 1.0 weight percent oxygen and is dissolved in the solvent used in a proportion of from 0.01 to 1 part per part by weight of titanium tetrachloride. Recommended are all known rubberlike hydrocarbon polymers, homopolymers or copolymers, such as polyisobutylene, ethylene-propylene copolymer, polyisoprene, polybutadiene, ethylene-propylene-dicylcopentadiene terpolymers and styrene-butadiene polymer, which have been oxidized by the action of oxygen. The oxidation of such polymers may be effected by grinding for a short time at elevated temperature in an oxygen atmosphere, for example, or by prolonged storage in air.
Following the reduction of the titanium tetrachloride in the presence of the hydrocarbon polymer at temperatures not exceeding 0.degree. C., the suspension is heated for 4 hours to 95.degree. C. and the titanium(III) catalyst component is rinsed with fresh solvent.
The use of the rubberlike hydrocarbon polymer containing active oxygen atoms has the effect that a finely crystalline catalyst is formed. In practice, however, difficulties are encountered in connection with the production of the hydrocarbon polymers. The oxygen uptake occurs through the surface of the polymer and because of the high viscosity of the product is difficult to control so that there is no assurance that the oxygen will be homogeneously mixed with and dispersed in the polymer. In analytical tests, too, representative sampling for the purpose of determination of the oxygen content of the hydrocarbon polymer as a function of reaction time and temperature level is possible only to a limited extent. Also, it must not be overlooked in this content that the activity of the catalyst is impaired by the presence of oxygen-containing compounds.